Is closely related to sampling rate, the sound also changes pitch. More samples (read) per second, the sound gets shorter. We can vary that rate,įor example playing back a sound sampled at 22.05 kHz at 44.1 kHz. To a sound (for playback and other calculations). Rate as the rate at which we stored (sampled) the sounds, but it alsoĬan refer to the kind of internal clock that the computer uses with reference Remember that sometimes we refer to the sampling To play it backįaster, you might raise the sampling rate, reading through the samplesįor playback more quickly. Now think of altering the speed of a digital signal. The Pitch/Speed Relationship in the Digital World In Soundfile 5.15, the opposite effect is heard: the slower file sounds lower in pitch. The faster the sound is played, the higher the pitch becomes, as heard in Soundfile 5.14. In this case, pitch and time transformations are related. A creative and virtuosic use of this technique is scratching as practiced by hip-hop, rap, and dance DJs.Īn example of time-domain pitch shifting/speed changing. The sound is also much shorter: in this case, pitch is directly related to frequencytheyre both controlled by the same mechanism. What happens? You play the record faster, the needle moves through the grooves at a higher rate, and the sound is higher pitched (often called the "chipmunk" effect, possibly after the famous 1960s novelty records featuring Alvin and his friends). Why should this even be difficult? Well, consider trying it in the time domain: play back, say, a 33 1/3 RPM record at 45 RPMs. Its length without changing its pitch and, conversely, to change its pitch Technique are to use a sounds Fourier representation to manipulate Two of the most important ways that musicians have used the phase vocoder Numbers that have phase and amplitude, and each corresponds to a frequency.) Of our music function into a list of Fourier coefficients, which are complex (Remember, Fourier analysis is the process of turning the list of samples Signal, representing it as a series of amplitudes, phases, and frequencies,Īnd manipulating this information and returning it to the time domain. Vocoder actually comprises a number of techniques for taking a time-domain Speech made twice as long with a phase vocoderĬommonly used implementation of Fourier analysis in computer music isĪ technique called the phase vocoder. Speech made half as long with a phase vocoder Fourier analysis (especially the FFT) is the key to many current spectral manipulation techniques. Spectral techniques allow us to represent and manipulate signals directly in the frequency domain, often providing a much more intuitive and user-friendly way to work with sound. Recently, a great deal of work in filter design has taken place directly To describe spectral change by designing time-domain operations. (before the FFT became commonly used by most computer musicians), attempted Filtering techniques, at least classically To manipulating the frequency content of sounds: filtering, and a combination Chapter 5: The Transformation of Sound by Computer Section 5.4: Introduction to Spectral Manipulation
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